FIG. 1 illustrates the working principle of an eddy current inspection device for inspecting an electrically conductive workpiece 4. The device operates by flowing a primary electrical current through a sensor comprising a coil of wire 1 to induce a primary magnetic field indicated by 2. The primary magnetic field 2 in turn induces a secondary electrical field 3 in an adjacent conductive (e.g. metallic) workpiece 4 to be inspected. The secondary electrical field 3 in turn induces a secondary magnetic field 5, which interacts with the primary electrical current flowing through the coil 1. The change in electrical current in the coil 1 can be detected by measuring one or more of the impedance (Z), magnitude (|Z|) and phase (φ) of the current I and/or voltage V flowing through the coil 1, to thereby deduce the properties of the workpiece 4 near the surface. This change can be detected using equipment such as an ammeter, voltmeter or oscilloscope. For example, small defects at or near the surface will result in a reduced or increased impedance, magnitude or phase of the current relative to other areas.
The surface of the workpiece 4 can be inspected by rastering the sensor 1 across the surface, and measuring the electrical properties of the current in the coil as the device is passed along the surface. However, such a method is relatively slow.
One alternative is to provide an array of sensors. FIG. 2 shows one such array 10, which comprises nine sensors 12 arranged in first and second rows. Each sensor 12 comprises a coil of wire similar to that shown in FIG. 1, which is independently supplied with electrical current. A centre of the coil of each sensor 12 is spaced a distance d2 from the centre of an adjacent sensor coil in the next row, with the rows being spaced, and with the rows being staggered such that the centre of the sensor 12 in the first row is located mid-way between the sensors 12 in the second row at a resolution better than the physical size of the sensors 12 in view of the staggered rows. By moving this array 10 in a single direction X normal to the row orientation, a large area of the workpiece 4 can be scanned at once by monitoring the signals produced by each sensor 12, thereby reducing the time required to perform a scan. However, such arrays generally have a lower sensitivity and a lower resolution compared to rastered single sensors, which is limited by the spacing d2 in the example shown in FIG. 2. Such arrays also have a lower signal to noise ratio in view of the proximity of the adjacent sensors 12, and so must be carefully normalised and calibrated to identify the correct threshold for rejection. Such calibration generally has to be repeated frequently, and can account for a large proportion of total testing time.
One calibration method is to provide an electrically conductive test article having a surface similar to the component to be inspected, having a single defect comprising a calibration notch of known dimensions, and passing the array along the test article, past the notch at position p. The signal is monitored, and a threshold set accordingly. FIG. 3 shows a graph illustrating a signal magnitude (such as phase, amplitude or impedance) of a single sensor 12 of the array 10 as it passes the notch p. The gain of the monitoring device is adjusted such that the screen height is approximately 80% of the maximum signal (so that the monitor is not saturated in use), and a rejection threshold is set at, for example, 30% of the screen height (which has been found to be sufficiently greater than the signal to noise ratio for most cases). Such a method is described for example in Chapter 8 of “Eddy Current Array Technology”, published by Eclipse scientific.
However, it has been found that such calibration methods may result in some defects going undetected. For example, if the length of the defect normal to the direction X is less than the sensor spacing d2, the peak signal from any one sensor 10 may be less than the rejection threshold, and in such cases, the workpiece would be accepted despite having a defect. In order to improve the accuracy of the rejection threshold, multiple passes must be made, with the array being slightly repositioned for each pass. Consequently, such calibration methods are time consuming. Similar problems also occur in other multi-element sensor arrays.
EP0969267 describes an alternative method and apparatus for calibrating a prior method of calibrating an eddy current sensor array. The method comprises providing a metallic plate having a plurality of spaced notches and passing the sensors array across the notches. However, generally, more than one pass is required in order to fully calibrate the array.
The present invention describes a method of calibrating an eddy current array inspection device which seeks to overcome some or all of the above problems.